The present invention is related to circuits and methods for performing nonlinear signal compensation to correct pulse asymmetry.
In magnetic disk storage systems for computers, digital data serves to modulate the current in a read/write head coil so that a sequence of corresponding magnetic flux transitions are written onto a magnetic medium in concentric tracks. To read this recorded data, the read/write head passes over the magnetic medium and transduces the magnetic transitions into pulses in an analog signal that alternate in polarity. These pulses are then decoded by read channel circuitry to reproduce the digital data. One type of magnetic transducer which is widely used for reading digital data from a magnetic medium is a magneto-resistive (MR) head.
An MR head is a device whose resistance varies with the applied magnetic field. In this regard, the head is capable of converting magnetic field variations produced by a rotating track into a time varying voltage or current in an electrical circuit. MR heads offer many advantages over other types of magnetic transducers and, consequently, are increasingly being used in magnetic data storage systems. For example, MR heads are more sensitive than other types of read heads, such as thin film heads, and produce a stronger read signal. Also, MR heads have a better frequency response than other types of heads which use inductive coils as a sensing means. In addition, the read signal produced by an MR head is relatively insensitive to the relative velocity between the head and the medium, as is the case with other types of heads, because it is the level of the applied magnetic field which is sensed by an MR head and not the rate of change of magnetic flux lines through a coil. This is an advantage in systems where head/medium velocity may vary over a significant range. Lastly, because MR heads are not capable of writing data on a magnetic medium, magnetic data storage systems which use MR read heads must include a separate head to perform the write function. Using a separate head for reading and writing allows each head to be separately optimized for performing its singular task which can greatly improve the performance of a magnetic data storage system.
An asymmetric characteristic is illustrated in FIG. 1. This asymmetric characteristic can produce problems in the conversion of the magnetic field variations emanating from the medium into the time varying electrical signal. For example, the asymmetric nature of the MR head may cause the time varying electrical signal produced by the head to look nothing like the magnetic signal applied to the head. To overcome this problem, a bias current is generally applied to the head to move the quiescent operating point of the head to be more asymmetric of the resistance characteristic.
Thus, proper biasing of the magneto-resistive element is critical for symmetric performance of the magneto-resistive transducer. Ideally, the magneto-resistive element should be magnetically biased by a soft film such that its voltage outputs, which correspond to the transitions in the magnetic field signal stored in the magnetic storage medium, are symmetrical about a reference voltage. However, various factors result in asymmetry of the outputs. One of the factors is the microstructural and magnetic property variations of the soft film at a microscopic level on the wafer. Moreover, the soft film properties are not uniform across the wafer. The presence of residues and contaminants on the wafer surface before soft film deposition enhances the non-uniformities even further. All of these result in non-systematic variations in the aforementioned voltage output asymmetry between manufactured magneto-resistive read heads.
It has been noted that the degree of asymmetry varies and, correspondingly, there is a need to adjust the asymmetry in accordance with this variance.
The present invention provides a circuit that performs an inverse nonlinear compensation to remove a portion of the signal distortion and thus diminish the degrading effects on the bit error rate (BER). The present invention employs a feed-forward circuit which compensates the asymmetrical signal. The circuit of the present invention uses a gain transfer function that employs a Taylor series expansion to yield a quadratic equation with a second order coefficient modified by k which is digitally programmable. The second order term is used to produce an inverse distortion to the signal which can approximately eliminate the MR asymmetry distortion.